The Skinny on Stars' Weight Fluctuations

By gaining about 30 pounds for the movie "Monster" — and losing it in time for the film’s premiere — former model Charlize Theron (search) joined the ranks of Renée Zellweger (search), Robert De Niro, Minnie Driver and Sylvester Stallone, actors who've all eschewed special effects for real adipose tissue (search) — and then speedily sprung back to size.

But experts say these transformations can give average Americans the wrong impression — that they too can fatten up and lose the weight in a flash.

“They have people to help them gain and lose the weight. We don’t have the luxury of personal trainers and nutritionists,” said Boston College sociologist and body image expert Sharlene Hesse-Biber. "It’s a false illusion, like Hollywood."

To play prostitute/serial killer Aileen Wuornos, Theron, 28, ate Krispy Kreme doughnuts to put on about 30 pounds. But soon after the film wrapped, the former ballerina jolted back to her leggy-blonde status to begin filming the lead romantic role in her next film, "Head in the Clouds."

And Zellweger, who gained 30 pounds to play lovably pudgy Bridget Jones in 2001, was rail-thin in her 2002 movies “White Oleander” and “Chicago.” The spindly star recently plumped up again for the Bridget Jones sequel.

In most interviews, Zellweger has dismissed all the hoopla made of her weight gain and loss.

"When ["Bridget Jones's Diary"] was coming out, the question I was asked the most was regarding my weight,' she said. "I was followed around Heathrow (airport) by a guy who wanted to take a picture of my backside. I don't understand the obsession."

But Hesse-Biber is concerned that people might think they can “pull a Renée and lose the weight tomorrow — but in doing so they’ll probably gain 20 pounds."

And the ease with which stars seem to lose the put-on pounds also has the added effect of making some people feel worse about their own extra padding.

As Bridget Harrison, who writes about the singles scene for the New York Post, put it in a recent column: “How annoying is it that one minute Charlize Theron can gain 30 pounds for her role in the movie "Monster" — and the next minute she's a svelte goddess again? I can't even lose a few pounds. How is it that Renée Zellweger can transform herself — again — into big-bottomed Bridget Jones, without any doubt that she'll turn straight back into a stick figure?"

Fitness experts say the public should remember that actors gain and lose weight as part of their job, and unlike most Americans they have the time, resources — and sometimes a contractual obligation — to do it.

“Renée may need to be smaller for her next role. As it’s her job, she makes it a priority in her schedule," said L.A. personal trainer Gunnar Peterson, who has worked with Jennifer Lopez, Ben Affleck, Lucy Liu and other A-listers. "She might have someone able to prepare meals for her, serve food to her on the job. She’s not relegated to a 30-minute lunch break where there’s only [snack] machines downstairs.”

Not to mention that gaining and losing significant heft over a short period of time is taxing on the body.

However, that's not the whole skinny. Peterson says shrinking stars may help convince some cynical dieters that weight loss is indeed possible.

After all, De Niro famously piled on the pounds for "Raging Bull" in 1980, but lost the girth later — and added major muscles for "Cape Fear" a decade later. Stallone, known for his physique, gained a gut for "Cop Land" and lost it again. And Driver made her first major big screen debut plumped up in "Circle of Friends," but has been svelte ever since.

“I would use that as a big motivator," said Peterson. "If they can drop 30, so can you. But take the short-term time component out.”

And New York City dietician Meg Corcoran doubts the average person is even fooled into thinking they could possibly "pull a Renee."

“The public is not so easily swayed. They know the stars have personal trainers," she said. "Anyone who has weight to lose knows it’s difficult and takes discipline.”